“Naturalized epistemology” is a recent attempt to transform the theory of knowledge into a branch of natural science. Traditional epistemologists object to this proposal on the grounds that it eliminates the distinctively philosophical content of epistemology. In this thesis, I argue that traditional philosophers are justified in their reluctance to accept naturalism, but that their ongoing inability to refute it points to deeper problems inherent in traditional epistemology. I establish my thesis first by critiquing three versions of naturalism, showing that each version either fails to implement its objectives consistently or starts from dubious objectives. “Optimistic naturalism” attempts to use naturalistic methods to give positive answers to traditional epistemological questions (such as by affirming that our beliefs can be justified). This fails because the form of traditional questions requires the concept “belief,” a concept that I argue naturalistic methodology cannot countenance (because of the severe constraints of naturalistic semantic theory). “Pessimistic naturalism,” the version advocated by Quine, is more consistent with naturalistic methodology, but fails because hidden behind its conclusions are assumptions (such as the underdetermination thesis) that cannot themselves be naturalized in the full context of scientific evidence. A final attempt to naturalize epistemology, deflationary naturalism, dispenses with traditional questions entirely, urging that the terms “knowledge” and “justification” be taken as mere pragmatic devices whose use can, nevertheless, be studied scientifically. Curiously, this view fails to take account of important facts about the actual use of those terms, as studied by psychologists. In the end, despite problems with each of these versions, traditional philosophers can learn something from their encounter with naturalism. In the course of showing how various naturalistic doctrines cannot themselves be naturalized, I identify the flaws in traditional epistemology (such as its reliance on a priorism, and its resulting inability to articulate a workable empirical foundationalist theory of justification) that make naturalism harder to challenge. These flaws must be corrected if philosophers are to preserve the autonomy of their discipline.